As far as I'm aware this file cannot be modified in the theme itself so you would have to modify...
/local/directadmin/data/templates/email_pass_change/index.html
Someone feel free to correct me if I'm wrong here but if you then update DirectAdmin you would lose these changes. To combat this I know you can copy other files into the custom folder (/local/directadmin/data/templates/custom) but I've never tried it with this file but I don't think this will work.
To do this you would need root shell access I would think so I guess the first question is, are you a reseller or admin?
Perhaps one of the DA support guys could confirm if this file is over-written in updates?